In the course of manufacturing paper and similar products, including paperboard and the like, it is well-known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such "fillers", the resultant paper can have a relatively poor texture due to discontinuities in the fibrous web. The said fillers are also important in improving the printing qualities of the paper, i.e. by improving the surface characteristics of same. The use of appropriate such fillers further, vastly improves the opacity and the brightness of a paper sheet of a given weight.
Among the materials which have thus found increasing acceptance as paper fillers are calcined kaolin clays. Materials of this type are generally prepared by calcining a crude kaolin, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. Reference may be usefully had to U.S. Pat. Nos. 3,014,836, to Proctor and 3,586,523, to Fanselow, which disclosures are representative of the prior art pertinent to the present invention.
Those properties which render a calcined kaolin pigment particularly valuable for use as a filler are also well known. These include a low abrasion value, and high brightness and opacifying characteristics. The low abrasion is significant in order to assure that the resultant paper product may be manufactured and processed using conventional machinery without damaging same. The brightness and opacifying characteristics are important in producing an acceptable paper sheet, one which incorporates whiteness, high opacity, good printability and light weight.
In addition to their use as fillers, calcined kaolin pigments have also found increasing application as paper coating pigments, wherein the characteristics of brightness and low abrasiveness remain of paramount importance.
The general relationship between calcining temperature and product brightness and abrasiveness is detailed in U.S. Pat. No. 3,519,453 to Morris et al which discloses in FIG. 3 that, as the calcining temperature was increased from about 1400.degree. F. to about 1800.degree. F. (760.degree. to 982.degree. C.), the G.E. brightness of the product increased about eight percentage points, and the abrasiveness approximately doubled.
Advantages of the present method are that it permits the production of higher brightness calcined kaolins, or of lower abrasiveness calcined kaolins, or it eliminates some of the beneficiation steps which must otherwise be applied to the feed to the calciner. Additionally, it permits the calcining treatment to be carried out at shorter residence times than previously used in the art, which is an advantage since the shorter the time of treatment, the lower the unit cost.
U.S. Pat. No. 3,586,523 to Fanselow et al teaches the significance of the purification or refining of the feed to the calciner, as in the Example where the crude kaolin is initially beneficiated by froth flotation, bleached with an oxidizing agent (potassium permanganate) and a reducing agent (zinc hydrosulfite), reference being made by the patentee in such respect to U.S. Pat. No. 3,353,668 to Duke. Such processing in Duke increased the G.E. brightness of the gray kaolin described therein from an initial value of 78% to a final brightness of 90.2% (refer to Example IV, the preferred embodiment).
U.S. Pat. No. 3,798,044 to Whitley et al discloses subjecting the calciner feed to a high intensity magnetic field, thereby removing magnetically susceptible contaminants, mainly iron and titanium minerals.
U.S. Pat. Nos. 3,014,836 to Proctor; 3,586,523 to Fanselow et al; and 4,381,948 to McConnell et al, disclose inter alia the importance, relative to the brightness and abrasiveness of the product, of the fineness of the feed to the calciner. These patentees achieved the desired fineness by fractionating the crude clay or by selecting a feed crude which is very fine to begin with. U.S. Pat. Nos. 3,171,718 to Gunn et al, and 3,798,044 to Whitley et al disclose delaminating the kaolin feed in order to achieve fine particle size feed.
U.S. Pat. No. 3,383,438 to Allegrini et al discloses burning the fuel in an external chamber of a rotary furnace, i.e. with a shielded flame, and admitting the resulting hot combustion gases to the calciner thereby eliminating the formation of vitreous particles which otherwise increase the abrasiveness of the calcined product. In effect the calciner is a convection section of a furnace.
As is known, calcination of kaolin clays may be carried out at temperatures above about 842.degree. F. (450.degree. C.), for example above 1652.degree. F. (900.degree. C.) but below 2012.degree. F. (1100.degree. C.)--to avoid the formation of crystalline abrasive mullite--e.g. at about 1850.degree. F. (1010.degree. C.) to about 1925.degree. F. (1052.degree. C.). The heating is carried out for a period which eliminates the hydroxyl groups of the kaolinite structure, and the crystalline structure of the kaolinite is destroyed. During calcination, the kaolin undergoes a well-defined endothermic reaction associated with dehydroxylation (also referred to as loss of water) when the clay temperature reaches about 1350.degree. F. (732.degree. C.). This results in a material usually referred to as metakaolin. If the clay temperature is further increased, the metakaolin undergoes a characteristic exothermic reaction at about 1800.degree. F. (982.degree. C.).
U.S. Pat. No. 3,941,872 to Puskar, discloses the sequential heating of a kaolin feed: first heating under a reducing atmosphere to a temperature in the range of 1400.degree. F. to 2200.degree. F., and thereafter heating the kaolin in, as termed by the patentee, "a conventional oxidizing atmosphere" to a temperature at least as high as the temperature to which it had been heated under a reducing atmosphere. Such sequential heating produced a product of higher brightness or of lower abrasiveness than was achieved when the oxidizing atmosphere was used throughout. The only enabling disclosure provided concerns heating a clay in the presence of carbon, employing an electrically heated muffle furnace and restricting access of air to some of the trays in the tests by closing those trays, thereby to provide a reducing atmosphere. Standard furnaces are usually direct-fired in that the burner combustion gases circulate directly over the charge; occasionally the flame may be permitted to impinge on the charge. Common examples are rotary kilns and open-hearth furnaces. On the other hand, muffle-type furnaces are frequently employed when the charge requires special atmospheres, see Chemical Engineers' Handbook, Perry and Chilton, McGraw-Hill, Fifth Edition, 9-33, 20-26. Puskar discloses that the carbon may be present as an additive. Alternatively, at least a portion of the carbon may be an indigenous impurity in the clay. Domestic (Georgia) gray kaolin is given as an example of a carbon-contaminated clay which may be used without supplementary addition of carbon in the practice of the preferred embodiment, see column 2, lines 54-61. It may be noted that an earlier patent, U.S. Pat. No. 3,586,523 to Fanselow et al discloses the calcination of "gray" kaolin by conventional methods, i.e. in an oxidizing atmosphere. Also, U.S. Pat. No. 4,034,058 to Jameson et al discloses a process of chlorinating calcined kaolin in which carbon optionally is added to the clay during chlorination at elevated temperatures to facilitate the formation of a volatile iron chloride.
Since Puskar requires a reducing ambient gas for the first stage of a two-stage calcination, it is worthwhile to consider whether the method is practicable for commercial operation.
In general, a furnace that obtains its energy by burning fuel, e.g. a hydrocarbon such as oil or, preferably, a gas fuel, operates with excess air, see U.S. Pat. No. 4,321,130, Example 1, column 6, lines 43-46, where it is shown that the stack excess air is 10% over stoichiometric for completely burning the fuel gas and see column 8, line 15, where a system using 15% excess air is noted. U.S. Pat. No. 4,332,546 describes a gas seal for a furnace using gas turbine exhaust and make-up air flow as combustion air. The events following a gas turbine trip are illustrated in FIG. 6 and described at column 7 lines 46-58, where it is shown that the minimum total air flow to the furnace remains above stoichiometric requirements at all times. It is stated at column 1, lines 54-64, that in the event that a gas turbine trips, one concern is that the furnace may become starved for oxygen temporarily. The resulting build-up of a high concentration of hydrogen or hydrocarbons in the furnace atmosphere followed by a sudden surge of oxygen, has the potential for causing an explosion and fire. Thus, a furnace can be operated safely with an excess of oxygen over the stoichiometric amount needed to react completely with the fuel. It therefore appears that a furnace burning a mixture of air and fuel is not amenable to providing a flue gas which is a reducing atmosphere having a deficiency of oxygen, i.e. excess fuel, but rather provides an oxidizing atmosphere. Neither is it suitable for supplying sequentially a reducing and then an oxidizing atmosphere. However, such furnaces, in particular wherein the flue gas is in direct contact with the charge, are highly desirable for calcining kaolin. One example is the rotary calciner in which the flame is shielded, of Allegrini et al, supra. Another preferred type is the multiple-hearth furnace known under various names, e.g. Herreschoff.
In a general design of a multiple-hearth furnace as described in Perry and Chilton, Ibid, 20-48, 49, a number of annular-shaped hearths are provided, one above the other. There are rabble arms on each hearth driven from a common center shaft. The feed is charged at the center of the upper hearth, the arms move the charge outward to the periphery where it falls to the next hearth, here it is moved again to the center and the procedure is repeated down the furnace. The hollow center shaft is cooled internally by forced-air circulation. Burners may be mounted at any of the hearths and the circulated air is used for combustion of the fuel. These furnaces handle granular material, thus are highly suitable for calcining kaolin in particulate form, and provide a long countercurrent path between the flue gas and the charge material.